Chemical weapons are reviled around the world. Even Adolph Hitler would not use them on the battlefield in World War II. So why would anyone resort to using them in Syria, especially in urban areas, knowing the world's attention was fixed on the fighting there?

We don't have a lot of hard data on what's happening in Syria right now, but the chances are very high that the Assad regime has used chemical weapons on rebel forces, especially in the southern areas of the nation—U.S. and Israeli intelligence saw Syria move chemical weapons from known stockpiles before the reported attacks of this month, unnamed sources told The New York Times. But it is also probable that members of the Syrian opposition—some part of a group designated by the United Nations as transnational terrorists—would use chemical weapons on civilians to drag the Western powers into an air war that might end in regime change, as happened in Libya. Remember, President Obama declared a "red line" condition for intervention—if the Assad regime uses gas, America has to do something. So the warships and long-range aircraft are poised to strike.

Given the world's aversion to nerve gas, and the American threat to intervene, why did Assad use it anyway? There are three reasons.

(1) Nerve Gas is Effective

Poison gas can help to change the tide of battle, especially when the military using it needs a breakthrough. The battle of Otaybeh, east of Damascus, is a good example. In late 2012, rebels surged into the town and were using it as a gateway to funnel weapons shipped from Jordan. (Jordan is reported to have a base where Saudi Arabia coordinates support for rebels. The CIA is also there, according to published but unverified reports, but is not shipping any significant weaponry.)

The government counterattacked in 2013, and the rebels held out for 37 days of punishing air strikes and artillery fire. The bombardment included chemical weapons in two instances, the rebels claimed. Medics on the ground said the chemical was an organophosphate, and the Assad regime is known to have sarin gas, which fits that report.

Sarin is particularly deadly in urban combat. It is easy to make into a gas that can penetrate bunkers and buildings. The gas is just a little heavier than air, so it hugs the ground. It has no odor or color, and so it strikes with no warning. A pinprick-size drop can kill an adult. Victims shown in activist videos show some of the symptoms of limited exposure: extreme weakness, rapid but labored breathing, watering eyes, nasal leaking, and drooling. Without treatment, the victims will choke to death or their hearts will stop. But sarin dissipates after a couple of hours, meaning the army that fires it can move into an afflicted area with no risk or without cumbersome protective gear.

Shortly after the gas attacks, the government forces moved in and retook Otabeh, perhaps reinforcing Assad's belief in chemical attacks.

(2) Winning the Proxy War

There has been hard fighting across the nation, including in the northern city of Aleppo, and the central city of Homs. But these places have not been the site of most of the chemical attacks. There have been just four reported attacks in these cities, and at least half of those have been disputed by experts on the ground.

An analysis of the chemical weapons attacks in Syria shows a definite pattern of use in locations south and east of Damascus. Even before the August attacks in the districts of Arbeen, Zamalka, and Ein Tarma that put the United States on its current war footing, chemical weapons attacks were reported in similar Damascus suburbs. There has to be a reason why.

There is something else happening in the south and east of Damascus—the Saudi Arabian government is establishing those areas as a hub of its rebel-support operations. These towns are vital because they remain a solid pipeline of weapons from Jordan into Syria. Antitank missiles are a good example of the way these foreign weapons can aid rebels. Reports from the rebels in mid-August indicate that they are starting to use Konkurs antitank weapons in the suburbs of Damascus.

The battle between Assad and the rebels is now a full-blown proxy war. Iran and Russia are on one side, and Saudi Arabia, Europe, and the United States are aligned on the other. Syria's chemical attacks are a message to the Saudis that the regime will do whatever it thinks it must to shut down enemy operations near its capital city.

(3) Not Afraid of Air Strikes

In Libya, the rebels were a coherent force facing a regime that did not have the full backing of international powers. Syria has the active backing of Iran and the material and diplomatic support of Russia, plus help from groups such as Hezbollah streaming into the country. Anyone who thinks Syria will fall as easily as Libya is delusional.

To maintain that strength, however, Syria needs its allies to stand behind it. And that explains why they're not afraid of using chemical weapons even if the U.S. responds with air strikes. In fact, the best way for Syria to maintain its alliances might be to encourage a strike by NATO—one that is not sanctioned by United Nations. (The U.N. will not approve strikes because China and Russia have Security Council veto power.) In this scenario, Syria can attack with chemical weapons and still claim to be the victim when the U.S. attacks without the U.N.'s say-so.

What Syria could gain from being attacked is more help from its friends, including sophisticated anti-aircraft weapons such as Russia's S-300, a renewed effort at intelligence gathering aimed at the U.S. and Saudis, and more diplomatic support. And if the crisis spreads, with Iran launching terror attacks, closing the Suez Canal, or taking some other action, then so much the better—the attention will shift away from Syria, which is exactly what the Assad regime wants.

What will it lose if the U.S. launches "limited air strikes?" Airplanes and airfields are easy targets for cruise missiles, as are command and control centers. But chemical weapons are not good targets. They kill innocents when they are bombed with cruise missiles, they are stored in protective bunkers, and there is no guarantee that anyone knows exactly where they are located. There are ways to destroy chemical weapons with air strikes, but it takes good intel and special bombs carried by aircraft—not missiles. But flying planes over there puts pilots in danger of Syria's antiaircraft defenses. Destroying the radar and missiles that could shoot down U.S. airplanes is not the job of a limited air campaign.

If Assad thinks he can endure the body blows and remain in charge, potentially with a strengthened position, that says a lot about why he's willing to thumb his nose at Obama and the world by using chemical weapons.

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